Upgrade factory_girl_rails to factory_bot_rails

[wtcruft]

From https://thoughtbot.com/blog/factory_bot:

As of the 4.9.0 releases of both factory_girl and factory_girl_rails, both gems will be officially deprecated. In the 4.9.0 version, while no functionality is changed, you’ll receive a deprecation warning asking you to switch to the renamed factory_bot gem during each test run.

Moving forward, all new changes will occur only on the factory_bot and factory_bot_rails gems.

https://github.com/thoughtbot/factory_bot/blob/4-9-0-stable/UPGRADE_FROM_FACTORY_GIRL.md